Physical relations define that a moving volume of air causes a pressure wave. Sound pressure level, or SPL, is what is perceived as loudness. SPL is given in units of decibels, a logarithmic pressure relation. This means that the volume of air that must be moved to produce a given SPL goes up exponentially with SPL. The range of SPL that humans can detect without physical pain is incredibly large. The difference in intensity between the quietest sound a human is able to hear (threshold of hearing) and the loudest sound a human can stand (threshold of pain) is literally the same relative difference as the difference in light intensity you would experience if you were standing next to a lighthouse in Massachusetts staring at the beacon and then looked across the Atlantic and saw a flashlight in London.
Loudspeakers use an electrical signal appropriate for audio to create sound. The amount of air that must be moved to obtain a given SPL is proportional to the inverse square of the frequency. This means that at low frequency the volume of air that must be moved to obtain a given SPL is very large when compared to the volume of air that must be moved to obtain the same SPL at high frequency. Loud and low means a very high volume of air movement. This is the reason that low-frequency transducers (woofers) are generally large compared to high frequency transducers (tweeters).
The volume of air moved by a piston (a loudspeaker can acoustically be thought of as a piston) is proportional to the piston area times the piston stroke. Normally large volumes of air are moved by utilizing a large piston area i.e., large diameter speaker cone, and moving the piston back and forth a relatively small distance. An added challenge exists when designing drivers with small piston diameters in that the area is proportional to the square of the diameter, meaning that the piston movement length must go up with the inverse square of the piston diameter to obtain a given SPL. Displaced volume increasing with the square of cone diameter can mean larger speakers.
Compact loudspeakers intended for low-frequency use are often designed by using a small diameter cone with a large maximum excursion.
FIG. 1-1 shows a typical conventional loudspeaker driver. With conventional loudspeaker technology, maximum excursion limit 112 possible for a given loudspeaker is usually limited by a design relationship between magnet 104 length and magnetic leakage 103. FIG. 1-1 illustrates that the excursion of voice coil former 101 is limited by back plate 102. FIG. 1-1 also shows that magnetic circuit 107 passes through the back plate 102 to complete the magnetic circuit. Usually the greater the distance between back plate 102 and top plate 108, the greater the area contributing to the magnetic flux leakage 103. This leakage may reduce the amount of magnetic flux in magnetic gap 109, which in turn may reduce the speaker strength, or force per ampere of current in voice coil 110. This reduced speaker strength decreases the efficiency of the speaker and its ability to produce acoustic output.
Magnetic leakage 103 and the associated reduction of flux in magnetic gap 109 can aggravate the overall long-excursion design problem because as the excursion becomes longer, the force needed to accelerate the moving mass through the excursion can become greater. This situation can create a need for high speaker strength, hence a large amount of magnetic flux in order to avoid requiring a large current to obtain the requisite force. A large current will tend to increase heat and could even overheat the voice coil 110.
As magnetic leakage 103 becomes greater with increasing excursion limit 112, a larger magnet 104 is usually used to create the needed flux in magnetic gap 109. The larger magnet 104 usually increases the weight and cost of the speaker, which is a commercial disadvantage. A further magnetic penalty for loudspeakers with long excursion can derive from the fact that spider 105 and surround 106 usually also have limited excursion ability. Spider 105 and surround 106 perform functions including keeping voice coil former 101 centered radially in magnetic gap 109. If voice coil 110 makes contact with top plate 108, or magnet 104, the voice coil will likely be destroyed. The nature of spider 105 and surround 106 devices dictates that the longer the excursion capability is, the smaller the radial stiffness and ability to center voice coil former 101 in magnetic gap 109. Therefore, as excursion limit 112 becomes longer, magnetic gap 109 often becomes larger to avoid damage to voice coil 110. The larger gap usually has an increased magnetic reluctance, which in turn can reduce the flux in magnetic gap 109. This reduction in flux can have the negative effect of reduced speaker strength. The need to dissipate heat can also create the need for a larger magnetic gap.
Almost every inherent relationship among loudspeaker parameters works against using a small diameter cone to move a large amount of air. A traditional approach to creating an overall loudspeaker assembly with high acoustic output at low frequency is to place the driver in a tuned-resonance enclosure that emits the sound or audio. A tuned-resonance enclosure can allow reasonably large excursion without a large applied force from voice coil 110 because mechanical resonant systems can produce a large output swing with a small force input at resonance. This can reduce the need for higher speaker strength and hence, can lower magnetic leakage. Drivers with moderate excursion limits and speaker strength are available from commercial sources for this purpose. The use of a tuned-resonance enclosure can have several shortcomings; however. Resonance tuning can require the enclosure to be of a specified volume, which can be determined by the physical parameters of the driver and the frequency response desired. In general, the greater the low frequency output, the larger the enclosure size. This can be a disadvantage in terms of weight and portability among other aspects. The low-frequency performance of a conventional driver utilized in a tuned-resonance enclosure can often be limited by excursion limits 112 of the driver. Even though the driver can produce a moderate excursion with a moderate amount of magnetic leakage 103, the magnetic leakage usually reduces the overall efficiency of the driver and can limit even the mid-frequency and high-frequency performance of the driver, which do not benefit from the resonant enclosure.
The mechanical resonant system may also have a phase-frequency response function between the applied voice coil force and cone acceleration, because stored energy in the system (pressure in the enclosure) pushes on the cone in one direction or the other depending on the circumstances. Since a phase shift is the same thing as a time delay, signals of different frequencies may be delayed by different times. This “time smear” can be termed group delay, and may be audible at low frequency where the wavelengths and delay time are long. Thus, the acoustic output of a tuned-resonance loudspeaker system can be phase-shifted (a phase shift is a frequency-dependent delay) from the input signal. This can produce an audible time delay between the low-frequency and high-frequency components of the signal, and can represent a kind of unwanted distortion.
Another more common failure mode of loudspeakers can include thermal overloading of voice coil 110, which can occur when the loudspeaker is operated at high volume for extended durations. Due to the random nature of audio signals, the user has no way of knowing if and when these conditions will exist, so the speaker system must generally be operated in a conservative fashion. Sometimes, operating the speaker in a conservative fashion causes the speaker to be larger and heavier than would otherwise be required in a given application.
The high-frequency performance of the conventional driver can also be reduced by the inductance of voice coil 110. Because the impedance of the voice coil usually increases with increasing frequency, the amount of current available from the amplifier (and consequently force from the voice coil) usually decreases with increasing frequency. There is also a distortion phenomenon in many ordinary loudspeakers due to the fact that the field induced by the current in the coil may add to and/or subtract from the field created by the permanent magnet. This may also be a source of audio distortion. Furthermore, a voltage is generated by the voice coil moving in the magnetic gap. This is sometimes called the motional voltage, and its polarity may be such that it tends to create a current that will create a force that accelerates the voice coil in the opposite direction from the velocity. The motional voltage may be directly proportional to the velocity and perhaps the speaker strength.
Another problem with a conventional loudspeaker driver is that the speaker strength can change with voice coil 110 position. As voice coil 110 moves out of magnetic gap 109, the number of windings immersed in the magnetic field decreases, thereby usually reducing the speaker strength. This can cause nonlinear distortion in the acoustic output. This nonlinear distortion can even represent the largest source of distortion in the entire sound reproduction system. In fact, nonlinear distortion created in this manner is frequently fifty times the total distortion produced by the remaining components in the signal chain combined. Non-constant speaker strength can also cause distortion. There can exist a design tradeoff between the efficiency of the speaker and the distortion created by the non-constant speaker strength function. By making the voice coil overhang 205a longer, the speaker will have more constant speaker strength over a wider range of motion, and hence less distortion. However, this longer voice coil 204b will often have increased mass and resistance compared to the shorter voice coil. This can reduce the efficiency of the speaker, since the resistive losses are often increased and the increased mass may result in decreases in sound pressure output for a given current through the voice coil.
FIG. 2-1 shows a conventional loudspeaker with a basket design. The surround 201 and spider 207 serve as the suspension for the cone 206b. The basket 201b provides the support structure for the suspension and magnetic circuit. Dust cap 206 prevents debris from accumulating in the magnetic gap 203b. The magnetic circuit is composed of the magnet 202b, top plate 202, back plate 203, inner pole piece 204a, and magnetic gap 203b. The force to move the cone is generated by current flowing through the voice coil 204b, a portion of which is immersed in the magnetic gap 203b. The force created by a given current, or speaker strength which causes a magnetically generated audio force is directly proportional to the strength of the magnetic field and the number of turns of wire immersed in it. FIG. 3-1 shows a cross-section of a typical dynamic loudspeaker. The basket 301b serves as a structure for mounting the surround 301, spider 307, and magnet structure 303c, which consists of the top plate 302a, magnet 302b, back plate 303, and inner pole piece 304. The annular magnetic gap 303b is formed by the top plate 302a and inner pole piece 304. For optimum magnetic performance, the magnetic gap 303b must be kept as narrow in the radial direction as possible. The magnetic gap must be somewhat wider in the radial direction than the voice coil assembly 305c in order to allow movement of the moving components without interfering with the magnetic structure 303c. The flying leads 305a carry current to and from the terminals 305b to the voice coil assembly 305c. 